In recent years, the size of semiconductor elements has been enlarged rapidly, and large thermal stress is formed between the semiconductor element and substrate. This is caused by the difference between the coefficient of thermal expansion of the semiconductor element composed of silicon and the coefficient of thermal expansion of the substrate based on an epoxy resin. The thermal stress is formed mainly at solder bump which connects the semiconductor element and the substrate, and underfill agents and the like are filled to relieve the stress. Although such a stress relaxation has an effect to some extent, it is necessary that the coefficient of thermal expansion of the substrate is substantially reduced to as low as that of the semiconductor element.
Aiming at decreasing the influence on environment, the solders have been changed from conventional lead-containing ones to lead-free type ones composed of tin, silver and the like. Since the lead-free solders need solder connection at higher temperature than the lead-containing solders, the substrate materials are required to have higher glass transition temperature than before.
To lower the coefficient of thermal expansion of an epoxy resin, a method of adding an inorganic filler is usually employed (see, for example, Patent Literature 1).
To raise the glass transition temperature of an epoxy resin, a method in which phloroglucinol is added to the epoxy resin (see, for example, Patent Literature 2); a method in which a curing agent having three or more phenolic hydroxyl groups is added to the epoxy resin (see, for example, Patent Literature 3); a method in which a cyanate ester resin is added to the epoxy resin (see, for example, Patent Literature 4); and a method in which a resin composition containing an epoxy resin, polyhydric phenol epoxy resin curing agent and polyethersulfone is added (see, for example, Patent Literature 5) and the like are provided.
Further, a method in which the adhesive property to a semiconductor element is enhanced by employing a thermosetting resin composition containing a bismaleimide compound and an epoxy resin (see, for example, Patent Literature 6); and a method in which high thermal resistance is attained by employing a liquid epoxy resin composition containing a liquid epoxy resin, curing agent and polyether compound, or by employing a composition containing an epoxy resin and a polyimide having a specific structure (see, for example, Patent Literature 7) are disclosed. However, the resin compositions disclosed in those literatures have problems in that the workability is poor due to insolubility in a solvent having low boiling point, and the thermal resistance is insufficient.
Aiming at improving the workability and thermal resistance, many heat-resistant resin compositions are provided, such as a thermosetting resin composition composed of a polyimide obtained by reacting a substituted guanamine compound and at least two unsaturated N-substituted maleimide groups (see Patent Literature 8); a cure type composition in which a polymer having an epoxy equivalent of 200 to 1,000 and an epoxy compound are dissolved mutually (see Patent Literature 9); an adhesive for electronic materials composed of a polyimide having a hydroxyl group and an epoxy resin (see Patent Literature 10); a heat-resistant resin composition composed of a compound having an amino group and phenol group, bismaleimide compound and epoxy resin (see Patent Literature 11); a resin composition composed of a polymer obtained from a bismaleimide compound and diamine, polyethersulfone resin and epoxy resin (see Patent Literature 12); a heat-resistant composition composed of a maleimide compound having a hydroxyl group with a specific structure and epoxy compound having two or more glycidyl groups (see Patent Literature 13); and a heat-resistant composition composed of an epoxy compound, bismaleimide compound and hydroxymaleimide compound (see Patent Literature 14). However, the resin compositions disclosed in those literatures have problems in that the molecular weight of polyimides to be blended is large, and the viscosity when mixed with an epoxy resin composition is too high.